ML0030 August 2009 Copyright 2009 Bitronics, LLC
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4.0 FIELD ADJUSTMENTS
The Bitronics WATT/VAR meters have been factory calibrated to display 000 +/- one digit
for zero signal input. PT and CT values are set to customer values if specified, or to 5:5 CT
and 1:1 PT otherwise.
4.1 Rescaling
One of the most powerful features of the WATT/VAR meters is the extreme ease of
rescaling the instrument on the bench or in the field. No calibrator is needed. Even though
the units are factory scaled to customer CT/PT ratios, these ratios may be changed in the
field as transformers are "tapped down". Rescaling should also be checked anytime the
meter is altered by the replacing of either the Electronics Module or the EPROM firmware.
Rescaling is simple and is carried out in the panel as follows:
1. With the WATT/VAR meter under power, remove the four screws holding the front
panel to the meter, then remove the front cover and gasket.
2. Flip the small toggle on the left of the meter UP for CT set (DOWN for PT set).
On the QTWIE2, the top display will show the present CT setting, and the bottom
display will show the present PT setting. The negative sign will be illuminated to
indicate the selected display. On WTWIE3 & RTWIE3 instruments only the selected
setting (CT or PT) will be shown on the top display. All CT ratios are assumed to
have 5 amps as their secondary maximum outputs. Therefore 10.0 on the display
corresponds to a 10:5 CT. PTs are represented by a ratio to 1, so 20.0 on the
display would correspond to a 20:1 PT. Powers of ten can also be set as the
decimal point moves from left to right by using the select button.
3. Index through the available CT/PT ratios by repeatedly pushing the select button.
A momentary push of the button will cause the display to increment to the next
power of ten, or to the next ratio. Holding the push button down longer than 1.2
seconds will cause the watchdog timer to reset the MCU, indicated by the display
test, followed by the version number of the instrument software, followed by the
display of the current CT & PT ratios. This is not a problem, and can be used to
reset the MCU, check for proper watchdog operation, check software version
number, or return to the current CT & PT ratio. Be sure to observe the proper
position of the decimal point. For ratios higher than 999, use a number expressed in
"kilo", that is, the ratios should have the decimal point following the first significant
digit. For example a PT of 4350:1 would use 4.35, and a CT of 3000:5 would use
3.0. Stop when you see the correct one. The table of CT ratios is listed below:
1.00, 10.0, 100.
1.10, 11.0, 110.
1.20, 12.0, 120.
1.50, 15.0, 150.
1.60, 16.0, 160.
2.00, 20.0, 200.
2.20, 22.0, 220.
2.40, 24.0, 240.
2.50, 25.0, 250.
